Slot fed horn radiator with protective radome having polarization and resistance wires embedded therein



SEMHZH RGQM Aug. 25, 1964 l. B. SERGE ETAL 3,146,449

SLOT FED HORN RADIATOR WITH PROTECTIVE RADOME HAVING POLARIZATION ANDRESISTANCE WIRES EMBEDDED THEREIN 2 Sheets-Sheet l FIG. I

INVENTORS P. S. WEBB I. B. SERGE gg 0.1

ATTORNEY I. B. SERGE ETAL 3,146,449 SLOT FED HORN RADIATORWITH'PROTECTIVE RADOME HAVING Aug. 25, 1964 POLARIZATION AND RESISTANCEWIRES EMBEDDED THEREIN 2 Sheets-Sheet 2 Filed Dec. 29, 1961 O O 0 ll 2ANGLE-DEGREES INVENTORS P. S. WEBB I. B. SERGE BY ATTORNEY United StatesPatent SLOT FED HORN RADIATOR WITH PROTECTIVE RADOME HAVING POLARIZATIONAND RE- SISTANCE WIRES EMBEDDED THERETN Igor B. Serge, Beverly Hills,and Peyton S. Webb, Sherman Oaks, Calif., assignors to The BendixCorporation, North Hollywood, Calif., a corporation of Delaware FiledDec. 29, 1961, Ser. No. 163,329 7 Claims. (Cl. 343-704) This inventionrelates to microwave antennas and more particularly to edge-slottedwaveguide radiators.

The Waveguide transmission line is a structure which is recognized aseasily adapted as a microwave antenna when a plurality of the slotsextend transverse to a face of the waveguide, and microwave energy isfed into one end of the waveguide. It is a well known practice tocontrol the radiation by varying the angle of orientation or position ofsuccessive slots with respect to the current paths in the waveguidewalls and thereby Vary the degree of coupling of the slots.

The slotted waveguide also has the advantage of simplicity andrelatively small size, but suffers from two disadvantages in that it isnot completely weather-sealed and consequently is not directly adaptableto marine or aircraft application, and furthermore has a rather widevertical pattern.

It is the general object of this invention to provide a microwaveantenna of simple design and easily fabricated and which exhibits a highdegree of directivity in both the horizontal and vertical planes. Forthe purpose of this specification, horizontally polarized energy will beconsidered that which is aligned in the same direction as the arrayitself, and vertically polarized energy will be defined as that which isperpendicular to the horizontally polarized energy as defined.

It is a further object of this invention to provide a slotted waveguideantenna which is sealed from the elements and which affords practicallycomplete suppression of cross-polarized energy.

These objects are all accomplished in accordance with the teaching ofthis invention, one embodiment of which comprises an edge-slottedwaveguide constituting the termination of a microwave transmissionsystem and the basic radiating element. The Waveguide is positionedwithin a recess in an elongated member constituting the main support ofthe antenna structure and having a pair of flaring portions forming theroot of a horn for the slot array. The length of the horn is extended bya dielectric radome member including internally metalized side walls andsealing end domes.

One feature of this invention relates to the elongated unitary memberconstituting the main support of the antenna and defining the horn.

Another feature of this invention evolves from the support memberincluding portions at the root of the horn for suppressingcross-polarized energy emitted from the waveguide slots. For the purposeof the present specification, when applicants refer to cross-polarizedenergy it is vertically polarized energy which is intended.

Still another feature of the invention relates to the radome structureproviding both a weather seal for the antenna and constituting the majorportion of the antenna horn.

Still another feature of the invention relates to the presence in theradome of vertically orientated heating Wires for maintaining theantenna ice-free in adverse conditions and providing simultaneouslyadditional crosspolarization suppression.

These and other features of the invention may be more clearly understoodfrom the following detailed descrip tion and by reference to the drawingin which:

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FIG. 1 is a front view of an antenna and rotor structure incorporatingthe invention;

FIG. 2 is a transverse section of the antenna along line 2--2 of FIG. 1;

FIG. 3 is an isometric showing of the radome portion of the antenna,FIG. 1;

FIG. 4 is a graphical representation of the horizontal radiation patternof the antenna, FIG. 1; and

FIG. 5 is a graphical representation of the vertical radiation patternof the antenna of FIG. 1.

FIG. 1 shows an antenna of this invention designed particularly formarine use. The structure includes a rotor housing 11 containing a rotormotor and mounted on a supporting shaft 12 which may be the foremast ofa marine craft. Secured to the upper rotating portion 13 of the rotor isthe antenna frame 14 including a channel member 15 best seen in FIG. 2.A waveguide 16 for feeding the antenna 10 extends through the rotorhousing 11 to a rotary joint within the channel 15 and unshown in thedrawing, through which microwave energy is conducted to ahairpin-shaped, horizontally extending waveguide section 20. The one leg21 of the hairpin waveguide member visible in FIG. 1 constitutes thefeed portion, and the second leg constitutes the radiating portion 22 asseen in the broken away section of FIG. 1. The front of the antennaproper is a dielectric curved window or radome 23 made of low lossmaterial, for example, resin-bonded fiberglass. End caps 24 of simi larmaterial seal the ends of the structure.

In the broken away portion of the radome, several slots 25 of theradiating leg 22 of the waveguide may be seen. The slots extend throughthe entire exposed face of the waveguide leg 22 and are in the order ofa small percentage of a wavelength in width. It should be noted in FIG.1 that the slots 25 do not extend normal to the length of the Waveguide22 but are inclined at slight angles with respect to its longitudinalaxis, and each of the slots is at a slightly different angle. Thisallows each of the slots 25 progressing from the feed end 21 of theantenna to intercept a predetermined portion of the current flowing inthe wall of the waveguide leg 22 and con sequently to have a controlledamount of radiation. It is well known in the art that a slottedwaveguide antenna may have a selected radiation pattern by proper choiceof slot-spacing and orientation.

The major component of the current flowing in the waveguide side walland intercepted by these slots 25 flows across the slots in a directionsubstantially parallel to the length of the waveguide. The radiatedenergy therefore is primarily polarized in a horizontal plane. Since theslots are not exactly perpendicular to the length of the waveguide, theenergy radiated also has a vertical plane component. This latter energyis desirably suppressed in order that the microwave system radiates andresponds only to horizontally polarized energy.

Substantially complete polarization in the horizontal plane isaccomplished by features of the structure described in connection withFIGS. 2 and 3.

Now referring in more detail to FIG. 2, the antenna proper may be seenas made up basically of a unitary hornshaped member including a recessbounded by three planar surfaces 31, 32 and 33. The planar surface 31and 32 are opposed and form the vertical positioning boundaries for thewaveguide leg 22, and the planar surface 33 constitutes the rear stopfor the waveguide. Between the opposed surface and the rear stop arerecesses 34 and 35 forming clearance openings between the horn memberand waveguide 22. The front edges or step portions and 41 of the opposedsurfaces 31 and 32 and the root portions 42 and 43 of the horn, alongwith the exposed major faces 44 and 45 of the waveguide, defineelongated cavities 46 and 50 having a width substantially great enoughto avoid restricting radiation from the ends of the slots and a depthsubstantially equal to an odd multiple of a quarter wavelength of themicrowave principal radiated frequency, thereby constituting a choke,tending to suppress vertically polarized radiation from the waveguideslots. The flaring portions 51 and 52 of the horn member 30 extendoutward from the root of the horn to limit the vertical radiationpattern. The flaring portion 51 and 52 of the horn member 30 extend onlya relatively short distance from the slotted front face of the waveguide22. Extending beyond the horn proper is the radome 23 for the antennaincluding planar Wall portions 53 and 54 forming extensions of the hornand a central dome aperture 55. The ends of the planar wall portions arerecessed in steps 60 and 61 at the outer lips of the horn, and the innersurface of the planar side walls 54 and 55 are metalized, for examplewith aluminum, so as to define a continuous conductive surface. Theaperture portion 56 of the radome 55 of course is unmetalized to allowthe passage of RF energy therethrough. By shortening or extending thedepth of the wall portions 53 and 54, the aperture 56 may be increasedor decreased, thereby controlling the beam width. In one specificembodiment the distance from the waveguide slots to the end of themetalized area of the side walls 53 and 54 is approximately six inches.The flare angle of the horn is approximately 30, and the beam width atthe half power points in the vertical plane is approximately 28.

It can be seen from FIG. 2 that the antenna proper is made up of threebasic elements: the edge-slotted waveguide 22 forming the radiator; thehorn-shaped support member 30 for the waveguide defining the root of ahorn and cross-polarization suppressor; and a radome 53 forming anextension of the horn and dome. The second element of this combination,the horn member 30, is preferably made of an aluminum or magnesium alloyextrusion having the cross-section appearing in FIG. 2. The extrudableshape of member 30 offers a prime advantage of extremely low cost. Thecritical dimensions of the entire structure are the spacing andorientation angles of the individual slots, and these are obtainable byaccurate milling processes available in the art.

The radome forming the extension of the horn is simply a vacuum-formedor otherwise molded resin-bonded fiberglass member having the two planarwalls 53 and 54 defining an angle of approximately 30. The angle of theside walls is not critical, and the only major restriction on thedimensions of the radome 53 is that the surface of the junctions 60between the support member 30 and the radome side walls 53 and 54 aresmooth so as to avoid reflections.

As may be clearly seen in FIG. 2, the antenna is supported by a simplechannel bracket secured to the circular plate 13 constituting therotating member of the rotor of FIG. 1. The channel shape of bracket 15provides a degree of mechanical protection for the feed leg 21 of thewaveguide 20.

In FIG. 3, another aspect of the radome is illustrated by the array ofdashed lines indicating the presence of embedded electrical conductors62 forming a vertically oriented grid across the face of the antenna.The ends 63 and 64 of the conductor 62 extend out of the radome 55 at asuitable point for convenient electrical connection. In practice theconductors are connected to a source of power to provide a heatingcurrent in the face of the radome to prevent the icing of the antennawhen it is subjected to subfreezing weather. The vertically orientedgrid per forms an additional function in that it provides furthercross-polarization suppression in combination with the chokes 46 and 50at the root of the horn. This efi'ect is best achieved when theconductors are spaced at minor percentages (significantly less thanone-half) of a wavelength apart. The net result of the two forms ofcrosspolarization suppression in actual practice is that the level ofvertically polarized energy radiated is suppressed 40 db.

The horizontal and vertical radiation patterns of the antenna of FIG. 1appear in FIG. 4 and FIG. 5, respectively. In FIG. 4 the ordinate axisis calibrated in radiated power in one direction, measured in decibels.The abscissa is calibrated in degrees in the horizontal plane from theaxis of the antenna normal to the center of the waveguide leg 22. Itshould be noted from FIG. 4 that the beam width at the half-power pointsis in the order of 2, giving an extremely narrow beam in azimuth. Thefirst side lobes are both down at least 28 decibels, and the major sidelobes down at least 25 or 26 decibels. The narrow beam is accomplishedby the orientation of the respective slots 25 as indicated above.

The vertical pattern shown in FIG. 5 indicates that the horn produces abeam width of approximately 28 at the half-power points. This beam widthis desirable for navigational applications.

In the description of the invention, reference is made only to a singlewaveguide array. It is recognized, however, that individual antennas maybe stacked vertically. Moreover, the cross-section of the support member30 may be modified to include two vertically displaced recesses for apair of waveguides and thereby achieve the same effect. Such amodification illustrates how the single support member 30 both literallyand figuratively forms the backbone of this invention. It supports andpositions the waveguide radiator, defines the vertical radiationpattern, and also provides cross-polarization suppression. The radomefurther limits the vertical pattern when it is internally metalized andadds to the crosspolarization suppression by the presence of theproperly oriented conductors. The cooperation of these elements with aslotted waveguide provides a simple, relatively inexpensive, andextremely effective microwave antenna.

Although for the purpose of explaining the invention a particularembodiment thereof has been shown and described, other modificationswithin the spirit and scope of this invention will occur to personsskilled in the art. The scope of this invention is only limited by theappended claims.

We claim:

1. In a microwave antenna including a waveguide section having aplurality of slots constituting a slot array:

a support for said waveguide comprising a unitary member having a lengthat least as great as the length of the slot array and including a recessfor positioning said waveguide with the slotted face thereof exposed,

said support including a pair of flaring wall portions extending outwardfrom said slot array,

the root of said wall portions terminating in a recess bounded on oneside by a side wall of said waveguide,

said recess having a depth substantially equal to an odd multiple of aquarter wavelength of the principal radiated frequency and therebyconstituting a crosspolarized energy suppressor,

and a radome of low-loss dielectric material including a pair of planarwall portions constituting an extension of the flaring walls of saidsupport and a central dome portion,

at least a portion of said planar wall portions of said radome beinginternally metallized to constitute an extension of the horn formed bythe flaring walls of said support.

2. A microwave antenna comprising:

a waveguide section including a plurality of slots extending generallytransverse to the length of one face thereof and forming a microwaveenergy-radiating array,

a support for said waveguide section including a unitary member havingan elongated recess for receiving said waveguide section and a pair offlaring wall portions embracing the slotted face of said waveguide andconstituting the root of a horn,

a radome of low-loss dielectric material including planar wall portionsconstituting extensions of the horn defined by said flaring walls ofsaid support, said radome including a central dome portion joining theoutermost ends of said planar portions to constitute an aperture,

and a plurality of electrical conductors extending across said centraldome portion of said radome in a direction transverse to the length ofsaid slot array, thereby constituting a cross-polarized energysuppressor.

3. The combination in accordance with claim 2 wherein said conductorsare embedded in said radome and extend beyond said dome portion intosaid planar wall portions.

4. The combination in accordance with claim 2 wherein said conductorsare resistance wires for heating said radome by the passage of currenttherethrough.

5. The combination in accordance with claim 2 wherein the inner surfaceof at least a portion of the planar wall sections of said radome aremetalized to constitute an extension of the horn defined by the flaringwalls of said support member.

6. A microwave antenna comprising:

a waveguide section including a plurality of slots extending generallytransverse to a face thereof and constituting an array of microwaveenergy-radiating apertures,

and a support for said waveguide section comprising a unitary memberhaving a length substantially equal to the length of said slot array anddefining a first elongated recess for positioning said waveguide withthe slotted face exposed,

said support including flaring walls embracing and extending outwardfrom the slot array,

said support further defining second and third elongated recessesbetween the root of each of said flaring walls and said waveguide, thesecond and third recesses constituting microwave chokes for attenuatingcross-polarized energy,

and a radome of low-loss dielectric material including a pair of planarwall portions constituting an extension of the flaring walls of saidsupport and a central dome portion.

7. The combination in accordance with claim 6 wherein said second andthird recesses are each bounded on one side by nonradiating faces ofsaid waveguide and by respective step portions of said support.

References Cited in the file of this patent UNITED STATES PATENTS2,298,272 Barrow Oct. 13, 1942 2,730,717 Katchky et a1 Jan. 10, 19562,814,038 Miller Nov. 19, 1957 FOREIGN PATENTS 642,825 Great BritainSept. 13, 1950 1,149,097 France July 1, 1957

2. A MICROWAVE ANTENNA COMPRISING: A WAVEGUIDE SECTION INCLUDING APLURALITY OF SLOTS EXTENDING GENERALLY TRANSVERSE TO THE LENGTH OF ONEFACE THEREOF AND FORMING A MICROWAVE ENERGY-RADIATING ARRAY, A SUPPORTFOR SAID WAVEGUIDE SECTION INCLUDING A UNITARY MEMBER HAVING ANELONGATED RECESS FOR RECEIVING SAID WAVEGUIDE SECTION AND A PAIR OFFLARING WALL PORTIONS EMBRACING THE SLOTTED FACE OF SAID WAVEGUIDE ANDCONSTITUTING THE ROOT OF A HORN,